This invention relates in general to acoustic energy absorbing structures and, more specifically, to an improved acoustic face sheet for such structures and methods of making the face sheet.
A number of different sound absorbing structures have been used as linings in ducts such as aircraft gas turbine engine housings to attenuate the noise generated by the engine. Typically, the sound absorbing structure may be a panel having an imperforate metal baking sheet to which is bonded a honeycomb structure with the honeycomb cells perpendicular to the backing sheet and having a perforated metal face sheet bonded to the opposite honeycomb face. As sonic energy generated by the engine impinges on the perforated sheet, it passes through the perforations and is captured in the honeycomb structure. Such structures are described, for example, in U.S. Pat. Nos. 3,493,774, 3,821,999 and 3,948,346. While these structures are relatively effective, they are also heavy. Typically, the perforated metal plate is aluminum weighing at least 1100 g/m.sup.2 in order to provide the necessary strength.
A considerable number of patents have issued describing face sheets made up of composite materials. For example, U.S. Pat. No. 4,112,164 describes an acoustic panel using a face sheet made from glass fibers in an epoxy resin matrix. While this is a very effective panel, the strength and weight of the glass fiber/epoxy face sheet are not optimum. U.S. Pat. No. 3,767,499 describes another method of making a face sheet in which ribbon-like bands of parallel fibers are laid down in a spaced relationship at angles of 0.degree., 90.degree., +45.degree. and -45.degree., with the band spacing selected to provide apertures of desired sizes through the sheet. This multi-layer structure, however, has greater than desired thickness and weight. Another multi-layer fabric composite face sheet is described in U.S. Pat. No. 3,502,171. Here, at least two sheets of biaxially woven fabrics are impregnated with resin and are superimposed with threads out of registry to provide apertures of selected sizes through the sheet. Again, this face sheet has greater thickness and weight than is desirable.
A lattice-type structure of resin impregnated carbon fiber bands is proposed in U.S. Pat. No. 4,092,453. While this structure has high strength and light weight, it is unsuitable for use as an acoustic panel face sheet because of the excessive thickness produced by this method.
Acoustic panel face sheets made from resin impregnated carbon fibers are described in U.S. Pat. Nos. 3,914,494 and 4,390,584. These patents teach weaving carbon fiber tapes in a biaxial manner to produce square or other quadrangular openings in the sheet. One or more layers of the biaxially woven fabric may be used These sheets have an excellent combination of light weight and high strength. However, the strength is concentrated in only two directions, along the warp and weft. These biaxial fabrics are sometimes difficult to shape to complex contours and may, where two or more layers are used, have undesirable thickness.
Thus, despite the significant contributions of a large number of inventors, there is still room for improvement in acoustic panel face sheets having an ideal combination of low weight, high and uniform strength, low thickness and superior formability.